The goal of this project is to produce "humanized" livers in large animal models such as sheep and pigs that can serve as a reliable source of significant numbers of functional human hepatocytes. The isolated human hepatocytes and/or "humanized" livers can be used for drug toxicity studies as well as for possible clinical use in patients with liver failure. Transplantation of human bone marrow (BM) CD34+ cells from normal donors of diverse genetic background into pre-immune fetal sheep/pigs can be used to generate human liver cells for use in drug toxicity studies (the aim of the present application), and from specific patients to produce livers and/or cells autologous to the patient (future goal requiring proof-of-principle studies). Feasibility data is presented demonstrating that the transplantation of CD34+ cells from human BM, mobilized peripheral blood, and cord blood into this non-injury pre-immune fetal sheep model resulted in the robust formation of relatively large numbers of human liver cells that synthesize and secret human albumin into circulation. In this human/sheep xenograft model donor (human) hepatopoiesis persists long-term (> 2years) without apparent loss of number or function. In the phase I effort we propose to determine: 1) the optimal gestational age of the sheep fetus, and 2) the optimal concentration of human bone marrow CD34+ cells for obtaining the highest percentages of human hepatocytes. Phase II effort will focus on efficient isolation, characterization, and maintenance of human liver cells from the "humanized" liver. [unreadable] [unreadable]